Some links were outdated/not working.

correct one url and one type of entry

fix typos, named correct arguments of function calls, etc.

With the new structure, /ref bibliography doesn't work any more
on the mainpage. However, this solution only circumvents the problem.

added dois/urls so that doxygen can generate hyperlinks to the publications

The changing constitutive relations task in exercise 1 previously was about
exchanging the EffToAbsLaw with a LinearMaterial law. It makes more sense to
replace the RegularizedBrooksCorey.
The description in the handbook was adapted accordingly.

Fill missing entries in feature table

*Delete unused entries

Include new features, add links to models and test problems

Should even work when they stick to version 3.0, but
not vice versa.

Testing links for implicit models